1. Field of the Invention
The embodiments of the subject matter disclosed herein generally relate to power generation systems and more particularly to turboexpanders.
2. Description of the Prior Art
Rankine cycles use a working fluid in a closed-cycle to gather heat from a heating source or a hot reservoir and to generate a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by transferring heat to a cold reservoir and pumped up to a heating pressure again to complete the cycle. Power generation systems such as gas turbines or reciprocating engines (primary system) produce hot exhaust gases that are either used in a subsequent power production process (by a secondary system) or lost as waste heat to the ambient. For example, the exhaust of a large engine may be recovered in a waste heat recovery system used for production of additional power, thus improving the overall system efficiency. A common waste heat power generation system operating in a Rankine cycle is shown in FIG. 1.
The power generation system 1 includes a heat exchanger 2, also known as a boiler or evaporator, a turboexpander 4, a condenser 6 and a pump 8. In operation, beginning with the heat exchanger 2, an external heat source 10, e.g., hot flue gases, heats the heat exchanger 2. This causes the received pressurized liquid medium 12 to turn into a pressurized vapor 14, which flows to the turboexpander 4. The turboexpander 4 receives the pressurized vapor stream 14 and can generate power 16 as the pressurized vapor expands. The expanded lower pressure vapor stream 18 released by the turboexpander 4 enters the condenser 6, which condenses the expanded lower pressure vapor stream 18 into a lower pressure liquid stream 20. The lower pressure liquid stream 20 then enters the pump 8, which both generates the higher pressure liquid stream 12 and keeps the closed-loop system flowing. The higher pressure liquid stream 12 then flows in to the heat exchanger 2 to continue this process.
One working fluid that can be used in a Rankine cycle is an organic working fluid. Such an organic working fluid is referred to as an organic Rankine cycle (ORC) fluid. ORC systems have been deployed as retrofits for engines as well as for small-scale and medium-scale gas turbines, to capture waste heat from the hot flue gas stream. This waste heat may be used in a secondary power generation system to generate up to an additional 20% power on top of the power delivered by the engine producing the hot flue gases alone.
Because of the concern that such hydrocarbon fluids can degrade and/or ignite if exposed directly to the high-temperature (˜500 degrees Celsius) gas turbine exhaust stream, measures need to be taken to limit the surface temperature of the heat exchanging surfaces in an evaporator which contains the ORC working fluids. A currently used method for limiting the surface temperature of the heat exchanging surfaces in an evaporator which contains the ORC working fluids is to introduce an intermediate thermo-oil loop into the heat exchange system, i.e., to avoid the ORC liquid circulating through the exhaust stack of the gas turbine. The intermediate thermo-oil loop can thus be used as part of an intermediate heat exchanger between the hot flue gas and the vaporizable ORC fluid.
As described above, the turboexpander 4 is used in a power generation system. The turboexpander 4 can be a centrifugal or axial flow turbine through which a high-pressure gas is expanded to produce work which can be used to generate power. An example of portions of a turboexpander 4 is shown in FIGS. 2 and 3, which are reproduced from U.S. Pat. No. 5,851,104(the '104 patent) the entire content of which is incorporated herein by reference. FIG. 2 shows a variable nozzle arrangement in a radial inflow turbine. The radial inflow turbine has a housing 102 with an annular inlet 104. A fixed circular plate 106 is positioned to one side of the annular inlet 104. The nozzle adjustment system is provided to the other side of the annular inlet 104. An adjusting ring 108 is arranged radially outwardly of a clamping ring 110. The adjusting ring 108 is able to rotate about the clamping ring 110 which is prevented from rotating by nozzle pivot pins 112 anchored in the fixed circular plate 106.
Vanes 114 are located about the annular inlet 104. These vanes are positioned between the fixed circular plate 106 on one side and the clamping ring 110 and adjusting ring 108 on the other. The vanes 114 are configured to provide a streamlined flow path there between. This path may be increased or decreased in cross-sectional area based on the rotational position of the vanes 114. The vanes 114 are pivotally mounted about the nozzle pivot pins 112. The relative positioning of the vanes 114 with respect to the clamping ring 110 is illustrated by the superimposed phantom line in FIG. 3.
In the '104 patent, the nozzle adjusting mechanism includes a cam and cam follower mechanism. Cam followers 116 are displaced laterally from the axis of the pins 112 and are fixed by shafts in the vanes 114 as shown in FIG. 3. The cam followers 116 rotate about the shafts freely. To cooperate with the cam followers 116, cams in the form of biased slots 118 are arranged in the adjusting ring 108. They are sized to receive the cam followers 116 so as to allow for free-rolling movement as the adjusting ring 108 is rotated. This arrangement of the vanes 114, cam followers 116, biased slots 118 and the adjusting ring 108 make the opening of the vanes 114 linearly dependant on a rotation of the adjusting ring 108. So, by adjusting the vanes 114, the amount of fluid allowed into the turboexpander 4 can be controlled.
In some cases turboexpander 4 can have multiple expansion stages, with each stage having a set of inlet guide vanes 114 to control the fluid flow. However, controlling the vanes 114 in the multiple expansion stages can change various parameters in the power generation system which can lead to difficulties in conventional power generation systems, e.g., the inability to regulate system pressure while optimizing power output efficiency.
Accordingly, systems and methods for more efficiently operating a power generation system are desirable.